To be able to adjust hybrid interface parameters and gain in a telephone circuit by means of a microprocessor on a line interface board in order to obtain good echo cancellation for telephone lines of different lengths as well as for certain test purposes, it is necessary to know the length of the respective telephone line.
One way of estimating the length of the line is to measure the line voltage. i.e. the voltage that appears across the line and the telephone set when a connection is established.
It is known to generate a pulse having a length corresponding to the line voltage. To get information about the line voltage, the length of this pulse is then measured by the microprocessor on the line interface board.
When a pulse length is to be measured, it is difficult to get at good resolution since the measurement period will be determined by the pulse length. The instruction cycle of the microprocessor has to be short relative to the shortest pulse length of interest and, moreover, the microprocessor cannot do anything else during that time.